The instant invention relates to a servo steering system for motor vehicles, and is of the kind comprising a steering worm shaft which is in operational connection with the steering wheel of the vehicle, closely penetrates a housing, and extends into a cylinder space provided within said housing. The steering worm shaft engages a steering gear nut which is generally axially displacable and drives a further gear member as, for example, by means of gear teeth. The steering gear nut functions simultaneously as a servo piston within the cylinder space, or, however, as seen in the axial direction, is fixedly connected with a servo piston. In this manner, the cylinder space is divided by the servo piston into two precisely separated working spaces, and the steering gear nut, either in a section which is formed as the servo piston of in an area remote from the sealing and guiding over of the servo piston, is provided with a lever on its outer surface. The lever moves in an axial direction and extends radially outwardly, and has an axial extent which corresponds at least with the lift of the servo piston. The lever engages a groove provided in the housing and arranged parallel to the axis of the nut. The length of the lever is measured in a manner so that the lever is able to perform minute tilting movements relative to the housing in cases where the steering gear nut, due to rotational momentum and increase of the steering worm grooves during a rotation of the steering worm shaft, is also caused to rotate. Thus, the tilting movements serve to operate at least one valve member being arranged in the housing vertical to the axial direction, in a manner so that the inflow of pressure to, or the outflow of the pressure from, a working space is controlled in such a way that the movement which is initiated by the steering worm shaft is supported in the axial direction, whereby the valve member, or the valve members, are provided with reaction areas which are stressed by the pressures in the working spaces or are in functional connection with respective reaction members so as to transmit a feeling of control or steering to the driver of the vehicle.
Prior art servo steering systems of this kind have been constructed with the working spaces which are divided in the cylinder space by means of the servo piston provided with essentially the identical effective areas. Accordingly, the valve member, or valve members, had the function of a 4/3-way valve; that is, when one operational space, or work chamber, was connected with a pressure-source, then the other operational space was in connection with an outlet, or vice versa. In a central position, either all conduits are blocked, or at least only partially connected to each other. The type of steering which thereby developed is also known as "four-edge steering" since the valve member, or the valve members, must be provided with four guiding edges.
In so far as the pressure supply is blocked during a center position of the four-edge steering, there may develop hydraulic knocks, sudden jerking movements in the steering system, and undesirable noise may develop during oversteering of a connection. In order to avoid such developments, it has already been proposed in other types of servo steering systems to utilize so-called "two-edge steering". "Two-edge steering" requires for functioning a differential motor -- namely, a servo motor with two differently dimensioned, large operational areas -- which are commonly of a 2:1 ratio. The smaller effective area is permanently stressed with pressure from the pressure source, while the larger effective area is connected interchangeably with the pressure source, or with an outflow, by means of a valve member which is provided with the function of a 3/3-way valve, such that, depending on its structure, in the center position of the valve member, this connection is blocked, or at least throttled.
In servo steering systems of the above-mentioned kind, it was formerly impossible, with small outer forces on the steered wheels of a vehicle, as it is known to be the case with icy road conditions, or fast highway driving, to steer without hydraulic servo support and with only a substantially pure mechanical power transmission.